4.4 Bonding in Alcohols and Alkyl Halides

H H H alcohols and alkyl halides are polar  = 1.7 D  = 1.9 D H H C O H H C Cl ++++ –––– –––– ++++ ++++ Dipole Moments

alcohols and alkyl halides are polar  = 1.7 D  = 1.9 D Dipole Moments

 +  –  –  +  +  – Dipole-Dipole Attractive Forces

 +  –  –  +  +  – Dipole-Dipole Attractive Forces

Boiling point Solubility in water Density 4.5 Physical Properties of Alcohols and Alkyl Halides: Intermolecular Forces

CH 3 CH 2 CH 3 CH 3 CH 2 F CH 3 CH 2 OH Molecular weight Boiling point, °C Dipole moment, D Effect of Structure on Boiling Point

CH 3 CH 2 CH 3 Molecular weight Boiling point, °C Dipole moment, D Intermolecular forces are weak. Only intermolecular forces are induced dipole-induced dipole attractions. Effect of Structure on Boiling Point

CH 3 CH 2 F Molecular weight Boiling point, °C Dipole moment, D A polar molecule; therefore dipole-dipole and dipole-induced dipole forces contribute to intermolecular attractions. Effect of Structure on Boiling Point

CH 3 CH 2 OH Molecular weight Boiling point, °C Dipole moment, D Highest boiling point; strongest intermolecular attractive forces. Hydrogen bonding is stronger than other dipole-dipole attractions. Effect of Structure on Boiling Point

++++ –––– –––– ++++ Figure 4.4 Hydrogen bonding in ethanol

Boiling point increases with increasing number of halogens CH 3 Cl -24°C CH 2 Cl 2 40°C CHCl 3 61°C CCl 4 77°C CompoundBoiling Point Even though CCl 4 is the only compound in this list without a dipole moment, it has the highest boiling point. Induced dipole-induced dipole forces are greatest in CCl 4 because it has the greatest number of Cl atoms. Cl is more polarizable than H.

But trend is not followed when halogen is fluorine CH 3 CH 2 F -32°C CH 3 CHF 2 -25°C CH 3 CF 3 -47°C CF 3 CF 3 -78°C CompoundBoiling Point

But trend is not followed when halogen is fluorine CH 3 CH 2 F -32°C CH 3 CHF 2 -25°C CH 3 CF 3 -47°C CF 3 CF 3 -78°C CompoundBoiling Point Fluorine is not very polarizable and induced dipole- induced dipole forces decrease with increasing fluorine substitution.

Solubility in water Alkyl halides are insoluble in water. Methanol, ethanol, isopropyl alcohol are completely miscible with water. The solubility of an alcohol in water decreases with increasing number of carbons (compound becomes more hydrocarbon-like).

++++ –––– –––– ++++ –––– ++++ Figure 4.5 Hydrogen Bonding Between Ethanol and Water

Density Alkyl fluorides and alkyl chlorides are less dense than water. Alkyl bromides and alkyl iodides are more dense than water. All liquid alcohols have densities of about 0.8 g/mL.